1. Field of the Invention
This invention relates to a circuit for integrating a quantity of light in an automatic control type flash unit.
2. Description of the Prior Art
The automatic control type flash unit performs the following functions:
(1) RECEIVING, BY A PHOTODIODE, LIGHT REFLECTED FROM AN OBJECT ILLUMINATED BY A FLASH DISCHARGE TUBE;
(2) CHARGING A CAPACITOR WITH A CURRENT CORRESPONDING TO THE QUANTITY OF LIGHT ENTERING THE PHOTODIODE; AND
(3) STOPPING THE FLASHING OF THE FLASH DISCHARGE TUBE WHEN THE CHARGING VOLTAGE OF THE CAPACITOR HAS REACHED A PREDETERMINED VALUE.
Usually, the quantity of light entering the photodiode is so great that the capacitor can be directly charged with the photocurrent from the photodiode. However, in special cases where the photodiode is contained within a camera body, the quantity of light entering the photodiode becomes smaller and it is therefore necessary to amplify the output of the photodiode. Unfortunately, an amplifier circuit usually has a disadvantage that it has in itself such error factors as voltage offset, temperature drift of bias current, etc. and these error factors appear at the output of the amplifier circuit. Therefore, mere amplification of the photodiode output by the amplifier circuit would be insufficient in that the error factors possessed by the amplifier change the point of time for stopping the flashing, thus preventing proper control of film exposure to be accomplished.